1. Field of the Invention
The present invention relates to air conditioners that employ water-cooled condensers. More particularly, the present invention relates to an enhanced economizer system for air conditioners that employ multiple refrigeration circuits and multiple water-cooled condensers.
2. Description of Related Art
Self-contained air conditioners are of a type typically used in mid to large size buildings, often on a floor-by-floor basis, and in other commercial and industrial settings. Such air conditioners are often referred to as "self-contained," as they can be packaged as an indoor unit: complete with one or more refrigerant compressors, condensers and evaporators for cooling air supplied to the building. The condensers take the heat that the evaporators absorb from the air and transfer the heat to cooling water that passes through the condensers. To remove the heat from the building, a pump circulates the cooling water between the unit's indoor condensers and an outdoor cooling tower where the heat is rejected to atmosphere.
For self-contained, water-cooled air conditioners and other types of air conditioners with multiple refrigeration circuits, the cooling water typically flows in parallel through the unit's condensers. Self-contained air conditioners often include two separate refrigeration circuits, so that when the heat load within a building is low, one or more of the circuits can be turned off to save energy.
In some self-contained air conditioners, a so-called waterside economizer is employed to save additional energy. A waterside economizer is simply a heat exchanger through which cooling water is circulated to directly cool the building's air with the same water that cools the condensers. Typically, economizers are only used when the cooling demand of the building can be satisfied by less than all of the air conditioner's refrigeration circuits and/or when certain low ambient temperature conditions exist. To use an economizer under such conditions, the cooling water has historically been piped to place the economizer coil in series with any condensers (i.e., in series with respect to the flow of cooling water). The economizer coil itself is disposed in the path through which the return airstream flows through the air conditioning unit. In other words, the economizer is in same airflow path as the air conditioner's evaporators, which cool the air for the building.
By incorporating an economizer in that manner, the cooling water piped to the air conditioner passes first through the economizer and then to an operating condenser (or to an inactive condenser if none are operating). Directing the building's relatively warm return air across the water-cooled economizer coil is an energy-saving way of providing a measure of cool air. Such cooling can, in fact, be accomplished even when none of the units' refrigeration circuits are operating. Under the right conditions, an economizer can provide 50% or more of the unit's cooling capacity without any of the air conditioner's refrigeration circuits being active. Such conditions may occur when the economizer receives cooling water (e.g., from a cooling tower) at a temperature that is significantly below the building's indoor return air temperature.
Although air conditioners with economizers provide significant energy-saving advantages, problems associated with pumping the cooling water can occur. For example, during periods when a unit's economizer is not being used, the condensers alone may only subject the water pump to a pressure head of about twenty-feet of water. However, when the economizer is activated and open to the flow of cooling water, the water passes in series through both the economizer and at least one condenser. Thus, if the economizer creates a pressure drop of about ten-feet of water, the pump must overcome a total pressure head of thirty-feet of water: twenty-feet for at least one condenser plus ten-feet for the economizer.
The increased head associated with flow through the economizer in such an arrangement has a significant influence on the selection and operation of the cooling water pump. If activating and deactivating the economizer causes a significant change in pressure for the pump, the pump and its motor may need to be oversized to meet the peak discharge pressure. In some cases, a pump may require relatively expensive and/or complicated two-speed, variable speed, or other methods of control in order to meet the varying pressure conditions. Varying pressure conditions can lead to additional expense relating to the need for some form of discharge pressure control for the pump when the economizer is inactive and isolated from the flow of cooling water.
Further, the cooling water pump in not always dedicated to serving just a single self-contained air conditioning unit. But rather, such a pump typically pumps cooling water to several individual units at the same time. This subjects the pump to even greater variations in pressure, as the various units independently activate and deactivate their respective economizers to meet their individual cooling demands. Thus, this further complicates the proper selection and operation of such a pump.
The need therefore exists for a modified, more energy efficient economizer arrangement for self-contained air-conditioning systems that employ multiple water-cooled condensers.